One of the major goals when collecting and condensing radiation, particularly visible light, from a source onto a target surface is the maximization of the flux density, or brightness, of the light at the target surface. Various configurations using on-axis elliptical and parabolic reflectors, and off-axis reflectors of various shapes have been used. Since the brightness of the image created at the target theoretically only can be conserved in an ideal optical system (and is reduced in a non-ideal system) it is impossible to increase the total flux at the target above the amount which is emitted by the source.
Specifically in the area of optical condensing and collecting systems which use reflectors, the fundamental system, exemplified by FIG. 1a, is comprised of a primary reflector 2 having a generally concave shape. Concave reflectors having a variety of shapes are known in the art, including spherical, paraboloidal, ellipsoidal, and torroidal reflectors. FIG. 1a specifically depicts a common ellipsoidal shaped concave reflector 2 which has two focal points 4 and 5. In such an ellipsoidal system, typically the source 1 of radiation will be placed near one focus 4, and the target surface 3, typically the input end of an optical fiber, homogenizer, or lens, is located near the other focus 5. One of the natural reflecting properties of an ellipsoidal shaped reflector is that light emitted at one of its foci will be collected and focused onto its other focus.
A technique commonly used by the prior art to combat the fundamental limitation that the total flux at the target surface must be at most equal to the flux emitted by the source is the use of an arc lamp as the source in combination with a retro-reflector. This combination takes the light emitted from one side of the arc lamp and redirects it with the retro-reflector back through the arc of the lamp. Since the absorption of the reflected light by the arc is very small, light emitted from the opposite side of the arc lamp when a retro-reflector is used is comprised of both the light radiating from the arc itself as well as the retro-reflected light. Thus, the total light flux emitted from the side of the lamp opposite the retro-reflector is effectively doubled. Other prior art methods have extended this concept by reflecting light from the arc back into itself multiple times, thus increasing the flux further as in U.S. Pat. No. 4,957,759 to Goldenberg et al.
As depicted by FIG. 1b, retro-reflectors have been commonly used in projection systems having an optical axis 17. A spherical retro-reflector 16 is placed behind the source 11, typically an arc lamp, with the arc 11a placed at the center of curvature 19 of the spherical retro-reflector 16. This orientation causes the light collected at the back of the source 11 to be imaged back through the arc 11a itself and be collected by condensing optics 18, such as lenses, at the front of the system. Such a retro-reflector 16 would effectively double the brightness being delivered to the condensing optics under the ideal circumstances, and in practice typically leads to around a 60% to 80% increase in flux density at the target surface 13.
To improve the flux density of the light delivered by the a reflector-based condensing system such as in FIG. 1a, a compound reflector system as shown by FIG. 2 has been developed by the prior art. Referring to FIG. 2, such a compound reflector system has on the opposite side of the source 21 from the target surface 23 an ellipsoidal primary reflector 22 which collects light from the source 21 located at a first focus 24 an reflects it toward a second focus 25. A concave spherical retro-reflector 26, situated with its center of curvature 29 being coincident with the first focus 24, collects a portion of the radiation emitted by the source 21 and reflects it back through the source 21 such that its effective flux density is nearly doubled. This retro-reflected light is then collected by the ellipsoidal primary reflector 22 same as the original light and delivered to the second focus 25, thus increasing the overall flux density at the target surface 23.
FIG. 3 shows another configuration of such a compound reflector system where the concave spherical retro-reflector 36 is placed behind the source 31 and the ellipsoidal primary reflector is placed between the source and the target surface 33. As with the compound reflector system depicted by FIG. 2, the source 31 is located near the first focus 34 of the primary reflector 32 and the center of curvature of the retro-reflector 36, and the target surface is placed near the second focus 35. Flux density at the target surface 33 in this case is also nearly doubled when compared to the case with no retro-reflection.
Although both the systems depicted by FIGS. 2 and 3 employ concave spherical retro-reflectors to increase the flux density at the target surface, the compound reflector system used in both is intricate and costly to manufacture. Furthermore, proper alignment between the lamp and the reflector is difficult. Thus, there remains a need in the art for an optimized system and method for optical condensing and collecting which increases the flux density of radiation emitted by a source toward a target surface which is simple and inexpensive to manufacture.